Friction drive systems for powering wheeled vehicles, such as bicycles, have existed for many years. These systems deliver power through direct contact with the wheel or tire of the vehicle. Typically, a motor is mounted in a fixed position adjacent to one of the wheels. The motor can then either drive the wheel through a secondary roller mechanism pressed against the tire or directly via tire contact with the rotating outer shell of an outrunner-type motor.
A contact surface on the rotating mechanism of the friction drive presses against the tire, thereby delivering mechanical power to the wheel. Friction between the contact surface and the tire keeps the tire from slipping (relative to the contact surface) and allows power to be transferred from the motor to the wheel. The force of friction equals the normal force (of the contact surface against the tire) times the coefficient of friction, which may be expressed as follows:Ff=μ*FN where Ff is the force of friction, p is the coefficient of friction, and FN is the normal force between the contact surface and the tire. The coefficient of friction is subject to change based on conditions like the weather. For example, when it rains and the tire becomes wet, the coefficient of friction typically drops significantly, reducing the force of friction for a given normal force. As another example, the coefficient of friction may be reduced if the tire becomes dusty or muddy.
When the coefficient of friction is suddenly reduced—for example, when the tire becomes wet after going through a puddle—slippage can occur between the tire and the contact surface. Such slippage can be dangerous, because it can result in sudden and unpredictable changes to the power delivered to the wheel. For example, after slipping, the tire may suddenly reengage (or “catch”) with the contact surface, causing a sudden increase in the power delivered to the wheel and in the resulting speed of the vehicle.
Known friction drive systems have difficulty responding to rapid changes in the amount of friction caused by weather (e.g., rain or snow), road conditions (e.g., dust or dirt), and other factors (e.g., loss of air in the tire). Some known systems use contact surfaces, such as sandpaper, having a high coefficient of friction to reduce slippage during changing conditions. However, such high-friction surfaces dramatically increase tire wear. Moreover, the sandpaper (or other high-friction surface) needs to be regularly replaced as it wears down, which is a tedious and time consuming process that requires regular monitoring by the consumer.
Another way to protect a friction drive system against changes in friction (e.g., due to changing road conditions) is to adjust the normal force between the contact surface and the tire. For example, a friction drive system could be configured to always provide a large normal force between the contact surface and the tire. However, continuously maintaining a large normal force requires more power due to tire churning, which drains the battery, and also increases tire wear.
In most known systems, the position of the contact surface relative to the tire is fixed when the friction drive system is installed. This fixed position, in turn, determines the normal force. In other systems, the normal force is set by a spring mechanism, gravity, or other biasing force. Still other systems provide a limited ability to adjust the normal force by manually reconfiguring the system, for example, by pulling a lever; however, such systems are difficult to control and typically require the user to stop the vehicle and dismount in order to change the settings.
None of these known friction drive systems provide a simple mechanism for adjusting the normal force. None of these known friction drive systems adjust the normal force dynamically in response to changing road conditions, weather, and the like. None of these known friction drive systems provide automatic traction control between the friction drive and the tire (or wheel). None of these known systems optimize the normal force to provide sufficient friction to avoid slippage while minimizing tire wear and maximizing battery efficiency.
Another problem with known friction drive systems is that they do not automatically disengage from the tire (or wheel) when the motor is no longer in use. Engaging with the tire (or wheel) when the motor is not actively providing power causes drag on the system, reduces efficiency, and slows the vehicle. Some known systems permit the user to manually disengage the motor by means of a lever or similar mechanism, which moves the contact surface away from the tire. However, such systems are inefficient because the user frequently forgets to disengage the contact surface or is unable to disengage (and reengage) the contact surface with optimal timing. Such known systems can also be dangerous; if the user reengages the contact surface when it is spinning at a high-differential speed compared to the wheel, the power delivered to the wheel (and the resulting speed of the vehicle) may change suddenly and unpredictably.
Accordingly, there is a need in the art for friction drive systems—and control algorithms for such systems—that can better adjust to changes in friction caused by road conditions, weather, and the like. There is a need in the art for an automatic traction control system for a friction drive that avoids slippage while minimizing tire wear and maximizing battery efficiency. There is also a need in the art for a system and method of automatically disengaging and reengaging the contact surface of a friction drive with the tire (or wheel) of a wheeled vehicle in a safe and efficient manner.